Euler gaussian bump flow solver case

[alex-fricker]

PR to migrate the euler gaussian bump to the flow solver framework, changes include:

• All gaussian bump based test cases (with the exception of euler_gaussian_bump_adjoint) now use FlowSolver gaussian_bump flow case
• Added gaussian bump subsection to parameters_flow_solver for grid generation parameters
• Added parameters for bump height, channel length, and channel height
• Moved num_refinements parameters for naca0012 and gaussian_bump flow cases to parameters_flow_solver (from parameters_grid_refinement_study)

[dougshidong]

Nice. Thanks for cleaning up the header includes when they were not needed anymore.